January 9 2020

Ruby Tip: Local Variables from Unexecuted Code

by Brian Dunagan

Recently, I had a bug where I reassigned params in a Rails controller but only for development mode. However, when I pushed to production, I found params would become nil after passing this unexecuted code.

As it turns out, this is a feature of Ruby: “The local variable is created when the parser encounters the assignment, not when the assignment occurs”. (Ruby documentation)

> x 
NameError: undefined local variable or method `y` for main:Object
  from (irb):3
  from /Users/bdunagan/.rvm/rubies/ruby-2.2.5/bin/irb:11:in `<main>`
> x = 1 if false
=> nil 
> x
=> nil 
> p local_variables
[:x, :_]
=> [:x, :_]